Sean O'Donnell

Three energy variables predict ant abundance at a geographical scale

Energy theory posits three processes that link local abundance of ectotherms to geographical gradients in temperature. A survey of 49 New World habitats found a two order of magnitude span in the abundance (nests m−2) of ground nesting ants (Formicidae). Abundance increased with net primary productivity (r2=0.55), a measure of the baseline supply of harvestable energy. Abundance further increased with mean temperature (r2=0.056), a constraint on foraging activity for this thermophilic taxon. Finally, for a given mean temperature, ants were more abundant in seasonal sites with longer, colder winters (r2=0.082) that help ectotherm taxa sequester harvested energy in non–productive months. All three variables are currently changing on a global scale. All should be useful in predicting biotic responses to climate change.

Methoprene accelerates age polyethism in workers of a social wasp (Polybia occidentalis)

Abstract. Topical applications of the Juvenile Hormone (JH) analogue methoprene to 1‐day‐old adult workers of the highly eusocial wasp Polybia occidentalis (Olivier) (Hymenoptera: Vespidae) accelerate the rate of age polyethism. Longevity of laboratory‐reared wasps is negatively correlated with dose of topically applied methoprene. Doses of 25 μg methoprene or greater are lethal. Untreated wasps show marked age polyethism in the field. Age of first performance of acts in seven behavioural categories (in‐nest, transition to outside, non‐task on nest envelope, nest maintenance, foraged material handling, defence, and foraging) is negatively correlated with methoprene dose. Topical applications of methoprene accelerate age polyethism of highly eusocial bee and wasp workers, but do not have this effect on primitively eusocial bees and wasps, suggesting that JH control of age polyethism evolved independently in advanced species of Apidae and Vespidae.

Dominance and polyethism in the eusocial wasp Mischocyttarus mastigophorus (Hymenoptera : Vespidae)

Abstract Dominance interactions affected patterns of non-reproductive division of labor (polyethism) in the eusocial wasp Mischocyttarus mastigophorus. Socially dominant individuals foraged for food (nectar and insect prey) at lower rates than subordinate individuals. In contrast, dominant wasps performed most of the foraging for the wood pulp used in nest construction. Social dominance also affected partitioning of materials collected by foragers when they returned to the nest. Wood pulp loads were never shared with nest mates, while food loads, especially insect prey, were often partitioned with other wasps. Dominant individuals on the nest were more likely to take food from arriving foragers than subordinate individuals. The role of dominance interactions in regulating polyethism has evolved in the eusocial paper wasps (Polistinae). Both specialization by foragers and task partitioning have increased from basal genera (independent-founding wasps, including Mischo-cyttarus spp.) to more derived genera (swarm-founding Epiponini). Dominance interactions do not regulate forager specialization or task partitioning in epiponines. I hypothesize that these changes in polyethism were enabled by the evolution of increased colony size in the Epiponini.

Assured fitness returns favor sociality in a mass-provisioning sweat bee, Megalopta genalis (Hymenoptera: Halictidae)

Assured fitness returns models for the evolution of sociality emphasize the selective value of ensuring that offspring receive adequate parental care to reach maturity. If a member of a social group dies, it can accrue returns on investment in offspring through the efforts of surviving social partners. We provide evidence that in the mass-provisioning, facultatively social sweat bee Megalopta genalis, adult presence in the nest throughout brood development provides protection from ant predation. Nests with adults present were well protected, and brood in nests with adults removed suffered higher predation. Females in observation nests showed effective defensive behavior against experimentally introduced ants, and bees in natural nests repulsed naturally occurring ant raids. Megalopta nest architecture and behavior are such that the brood of several cooperating females can be defended with little additional cost relative to solitary nesting. The benefits of cooperative defense may favor group living in mass provisioning bees. Our observations and experiments suggest that parental care throughout brood development can be adaptive in mass provisioning species, supporting the predictions of assured fitness returns models.

Forager specialization and the control of nest repair in Polybia occidentalis Olivier (Hymenoptera : Vespidae)

SummaryWe measured patterns of individual forager specialization and colony-wide rates of material input during periods of response to experimental nest damage and during control periods in three colonies of the tropical social wasp Polybia occidentalis.(1)Most foragers specialized on gathering a single material. While active, foragers rarely switched materials, and most switching that did occur was between functionally related materials — prey and nectar (food materials) or wood pulp and water (nest materials).(2)Individuals differed greatly in activity level, here expressed as rate of foraging. Workers that foraged at high rates specialized on a single material in almost all cases. Specialized, highly active foragers comprised a minority (about 33%) of the working foragers in each colony, yet provided most of the material input.(3)Individual wasps that responded to experimental nest damage by foraging for nest materials did not gather food on days preceding or following manipulation.(4)On the colony level, nectar and prey foraging rates were not affected by foraging effort allocated to nest repair within days, or when comparing control days with days when damage was imposed. The emergency foraging response to nest damage in P. occidentalis did not depend on effort recruited away from food foraging.

Lifelong patterns of forager behaviour in a tropical swarm-founding wasp: effects of specialization and activity level on longevity

Abstract The relationships between foraging tenure and specialization on materials, and foraging tenure and foraging activity were measured for marked, known-age workers of the social wasp Polybia occidentalis . The purpose of the study was to assess differences in cost to foragers, in terms of risk of mortality, associated with material specializations and activity level, and determine whether ontogenetic changes in forager behaviour reflect these differences in cost, such that more risky patterns of behaviour are postponed until later in adult life. Polybia occidentalis workers terminated foraging at an average of 5·9 days after their first trip, independent of their age at first foraging, suggesting that this task entailed a high risk of mortality. Lifetime foraging activity was calculated as foraging rate (mean number of trips per hour), foraging effort (mean number of time-weighted trips per hour), and as the proportion of observation days that an individual was active during her foraging tenure. Foraging tenure was not related to foraging rate or effort, but had a strong negative relationship with the proportion of observation days during which an individual foraged. By the latter measure, risk of mortality increased with foraging activity. The length of foraging tenure was negatively correlated with the proportion of foraging effort devoted to food materials (nectar and insect prey), supporting the hypothesis that gathering food was riskier than gathering nest materials (water and wood pulp). Foragers that switched between food and nest materials exhibited no tendency to specialize on nest materials first and therefore foraging for riskier materials was not always performed later in adult life. These results are discussed in terms of possible factors regulating task performances at the individual level in social insects.

Effects of Experimental Forager Removals on Division of Labour in the Primitively Eusocial Wasp Polistes Instabilis (Hymenoptera: Vespidae)

Experimental forager removals were performed to assess the mechanisms by which Polistes instabilis colonies regulate their intake of nectar and water. Most foragers gathered nectar, while water was collected by a small number of fixated foragers. Removal of the most active water foragers led to decreases in water foraging, followed by recruitment of a single replacement water forager. Replacement water foragers were usually recruited from among the workers that had previously collected water at low rates. Water forager removals showed that some workers specialized on water collection, but these workers differed in their thresholds of response to colony need for nest cooling. Removal of the most active nectar foragers led to longer-lasting (one to three days) decreases in colony nectar collection rates, and resulted in replacement nectar foragers being recruited away from other foraging tasks or from nest tasks. Nectar forager removals were followed by increases in rates of dominance interactions among nest wasps; this response was not observed after water forager removals. Dominance interactions among workers appear to regulate nectar foraging in P. instabilis. The mechanisms of regulation of foraging differ among materials, and correspond to their maximum rates of collection, predictability of resources, and on the costs of short-term changes in supply to the colony.

RAPD markers suggest genotypic effects on forager specialization in a eusocial wasp

Abstract Genetic variability within insect societies may provide a mechanism for increasing behavioral diversity among workers, thereby augmenting colony efficiency or flexibility. In order to assess the possibility that division of labor has a genetic component in the eusocial wasp Polybia aequatorialis, I asked whether the genotypes of workers within colonies correlated with behavioral specialization. Workers specialized by foraging for one of the four materials (wood pulp, insect prey, nectar, or water) gathered by their colonies. I collected foragers on 2 days from each of three colonies and identified the material the foragers were carrying when collected. I produced random amplified polymorphic DNA (RAPD) markers from the genomic DNA of these foragers and estimated genotypic similarity of foragers based on sharing of variable RAPD marker bands. Contingency tests on 20 variable loci per colony showed statistically significant (P <0.05) biases in RAPD marker frequencies among forager types in the three colonies. Patterns of association of RAPD marker bands with specializations were constant in two colonies, but changed between collection days in one colony. RAPD marker biases suggest that division of labor among workers includes a genetic component in P. aequatorialis. Colony-level selection on variation in division of labor is a possible factor favoring the evolutionary maintenance of high genotypic variability (low relatedness) in epiponine wasp colonies and in other eusocial insects.

The role of male disease susceptibility in the evolution of haplodiploid insect societies

Heterozygosity at loci affecting resistance against parasites can benefit host fitness. We predict that, in haplodiploid species, haploid males will suffer decreased parasite resistance relative to diploid females. We suggest that elevated susceptibility in haploid males has shaped the evolution of social behaviour in haplodiploid species. Male susceptibility will select for behavioural adaptations that limit males’ exposure to pathogens and that limit male transmission of pathogens within and between colonies. The relatedness–asymmetry hypothesis that has been advanced to explain female–only workers does not make these predictions. We review the relevant evidence for genetic effects on parasite resistance in insects and summarize empirical evidence that relates to the haploid–susceptibility hypothesis.

Reproductive physiology, dominance interactions, and division of labour among bumble bee workers

Abstract.  Bumble bee workers (Bombus bifarius, Hymenoptera: Apidae) exhibit aggression toward one another after the colony begins producing female reproductive offspring (the competition phase). Workers in competition phase colonies must continue to perform in‐nest tasks, such as nest thermoregulation, and to forage for food, to rear the reproductives to maturity. Therefore, competition phase workers are faced with potentially conflicting pressures to work for their colonies, or to compete for direct reproduction. The effects of reproductive competition on worker task performance were quantified by measuring relationships of worker body size, reproductive physiology, and aggression with their rates of task performance. If worker division of labour was strongly affected by competition, it was predicted that fecund workers would avoid performing nest maintenance and foraging tasks, focusing instead on reproductive behaviour. Furthermore, it was predicted that fecund workers would dominate their nest mates, and that subordinate workers would perform nonreproductive tasks at higher rates. Worker aggression was associated closely with direct reproductive competition. Both aggression and brood interaction rates were related positively with ooctye development. Furthermore, foraging was associated negatively with ovarian development. However, in‐nest and foraging task performance rates were not associated with social aggression. The results support a partial role for reproductive competition in worker polyethism. Although worker aggression did not directly affect polyethism, reproductively competent workers avoided foraging tasks that would remove them from egg‐laying opportunities. Reproductively competent workers did perform in‐nest tasks, suggesting that these tasks entail little cost in terms of reproductive competition.